thermodynamics 2 e7

Thermo level 1\Thermo & Fluids Module\Thermo & Fluids Module - Thermo Notes Book 2-Dec 07 John Withers - December 2007

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THERMODYNAMICS & FLUIDS (Thermodynamics level 1\Thermo & Fluids Module -Thermo Book 2-Contents-December 07.doc)

UFMEQU-20-1

THERMODYNAMICS NOTES - BOOK 2 OF 2

Students must read through these notes and work through the various exercises in their own

time in parallel with the course of lectures.

Thermodynamics is the study of the relationships that exist between the

properties of a gas or a vapour and the transfer of heat and work energy to or

from that gas or vapour.

Contents

Page Number

Formulae & data 2

Non-flow processes – Gases 4

Exercise 9

Steady-flow processes – Gases 13

Exercise 16

Steam tables 24

Exercise 28

Non-flow processes – Steam 29

Exercise 31

Steady-flow processes - Steam 35

Exercise 38


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FORMULAE & DATA GENERAL

Density ρ = mass / volume = m/V Specific volume v = V/m ρ = 1 / v

ρ = 1000kg/m3 for water ρ = 13600kg/m3 for mercury 1m3 = 1000litres

KE = ½mC2 PE = mgZ g = 9.81m/s2 1bar = 105 N/m2

____________________________________________________________________________

PRESSURE:

Pressure p = force / area = F / A Absolute pressure p = pgauge + patmos

Note that hp is sometimes represented by ‘z’

Atmospheric pressure patmos = ρghb where hb is the barometric head

Standard atmospheric pressure patmos = 1.01325bar 1bar = 105 N/m2

_________________________________________________________________

FOR GASES

ToK = ToC + 273 R = gas constant

pV = mRT v = V/m = RT/p V = mv ρ = m/V = p/RT

For any process, change in internal energy: (U2 - U1) = mcv(T2 - T1)

For any process, change in internal energy per kg: (u2 - u1) = cv(T2 - T1) U = mu

For any process, change in enthalpy: (H2 - H1) = mcp(T2 - T1)

For any process, change in enthalpy per kg: (h2 - h1) = cp(T2 - T1) H = mh

Gas constant: R = cp - cv γ = cp / cv

For any process: p1V1/T1 = p2V2/T2;

For a polytropic process:

p1V1n = p2V2

n and p1V1/T1 = p2V2/T2 and T2/T1 = (p2/p1)(n-1)/n = (V1/V2)

(n-1)

For an adiabatic process (Q = 0):

p1V1γ = p2V2

γ and p1V1/T1 = p2V2/T2 and T2/T1 = (p2/p1)(γ-1)/γ = (V1/V2)

(γ-1)

For constant volume heating: Q = mcv(T2 - T1) ( i.e. Q = U2 - U1)

For constant pressure heating: Q = mcp(T2 - T1) (i.e. Q = H2 - H1)

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FOR WET STEAM

u = xug + (1-x)uf

h = xhg + (1-x)hf or h = hf + xhfg

v = xvg + (1-x)vf or v = xvg approximately

_____________________________________________________________________________

NON-FLOW THERMODYNAMIC PROCESSES FOR GASES & VAPOUR S

NFEE: Heat in – Work out = change in Internal Energy

NFEE: Q - W = (U2 - U1)

NFEE per kg of fluid: q – w = (u2 - u1)

U = mu

Q = 0 for adiabatic processes

W = 0 when V = constant

W = p(V2 - V1) when p = constant

W = p1V1ln(V2/V1) when pV = constant

W = (p1V1 - p2V2)/(n-1) when pVn = constant (Polytropic process)

W = (p1V1 - p2V2)/(γ-1) when pVγ = constant (GAS undergoing an adiabatic process)

____________________________________________________________________________

STEADY FLOW THERMODYNAMIC PROCESSES FOR GASES & VAP OURS

SFEE:

Heat in – Work out = change in potential energy + change in kinetic energy + change in enthalpy

SFEE: Q - W = mg(Z2 - Z1) + ½m(C22 - C1

2) + (H2 - H1)

SFEE for 1kg of fluid: q - w = g(Z2 - Z1) + ½(C22 - C1

2) + (h2 - h1)

H = mh

Q = 0 for adiabatic processes

Mass flow rate &m= A1C1/v1 = A2C2/v2 or &m= ρ1A1C1 = ρ2A2C2

Power = w&m

____________________________________________________________________________


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NON-FLOW PROCESSES – GASES (Thermodynamics\ level 1\ non-flow gas.doc)

CLOSED SYSTEMS Non-flow processes take place in closed systems

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THE GAS LAWS

The gas laws given below apply to any process where the working fluid is a gas.

PV/T = mR = Constant pv/T = R for 1 kg R = cp - cv p1V1/T1 = p2V2/T2

U2 - U1 = mcv(T2 - T1) u2 - u1 = cv(T2 - T1) H2 - H1 = mcp(T2 - T1) h2 - h1 = cp(T2 - T1)

THE NON-FLOW ENERGY EQUATION

The NFEE shown below applies to all non-flow processes.

Q - W = (U2 - U1) or q - w = (u2 - u1) per kg

Q is the heat transfer into the system W is the work transfer out of the system

(U2 - U1) is the change in internal energy of the system

U2 - U1 = mcv(T2 - T1) or u2 - u1 = cv(T2 - T1) / kg for all gas processes

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WORK DONE W

All the work done expressions for non-flow processes involve the product (pV) in some form.

Thus for gases, (pV) can be replaced by (mRT) from the equation of state, pV = mRT

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Piston

Work output from system W

Heat transfer into system Q

CLOSED SYSTEM

CYLINDER & PISTON

Real continuous boundarywhen valves are closed

(thermo\cylinder.sdr)


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CONSTANT VOLUME PROCESS FOR A GAS

ISOMETRIC PROCESS Process law: p/T = constant from pV/T = constant with V = constant

Or p1/T1 = p2/T2 from p1V1/T1 = p2V2/T2 as V1 = V2

Q = mcv(T2 – T1) by definition of cv.

W = 0

U2 – U1 = mcv(T2 – T1)

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CONSTANT PRESSURE PROCESS FOR A GAS

ISOBARIC PROCESS

Process law: V/T = constant from pV/T = constant with p = constant

Or V1/T1 = V2/T2 from p1V1/T1 = p2V2/T2 as p1 = p2

Q = mcp(T2 - T1) by definition of cp

W = p(V2 - V1) or W = mR(T2 - T1)

U2 - U1 = mcv(T2 - T1)

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CONSTANT TEMPERATURE PROCESS FOR A GAS

ISOTHERMAL PROCESS A constant temperature process is known as an isothermal process.

Process law: pV = constant from pV = mRT with T = constant

Or p1V1 = p2V2 from p1V1/T1 = p2V2/T2 as T1 = T2

Also (V2/V1) = (p1/p2)

W = p1V1ln(V2/V1) or W = p1V1ln(p1/p2) ln means “log to base e”

Or W = mRTln(V 2/V1) or W = mRTln(p1/p2) as pV = mRT

U2 - U1 = mcv(T2 - T1) = Zero as T1 = T2

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POLYTROPIC PROCESS FOR A GAS

This is a general type of process.

Process law: pVn = constant i.e. p1V1n = p2V2

n

"n" is known as the Polytropic index and has a value which is normally between 1 and 1.5.

Thus two equations apply to a polytropic process when the working fluid is a gas:

The polytropic law p1V1n = p2V2

n which gives p2/p1 = (V1/V2)n ----------------(1)

The gas law p1V1/T1 = p2V2/T2 which gives T2/T1 = (p2/p1)(V2/V1) --------(2)

From (1) & (2) T2/T1 = (V1/V2)n(V2/V1) = (V1/V2)

n(V1/V2)-1 = (V1/V2)

n-1

i.e. T2/T1 = (V1/V2)n-1 ---------------------------------------------------------(3)

From (1) V1/V2 = (p2/p1)1/n

Substitute in (3) T2/T1 = [(p2/p1)1/n]n-1 or T2/T1 = (p2/p1)

(n-1)/n --------(4)

Combine (3) & (4)

T2/T1 = (p2/p1)(n-1)/n = (V1/V2)

n-1

Also W = (p1V1 - p2V2)/(n-1) or W = mR(T1 - T2)/(n-1)

And U2 - U1 = mcv(T2 - T1)

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ADIABATIC PROCESS FOR A GAS

An adiabatic process is a special type of polytropic process in which there is zero heat transfer,

i.e. Q = 0

A reversible adiabatic process is also known as an isentropic process, i.e. a process in which the

property entropy remains constant.

Adiabatic conditions can be achieved if either:

(a) The temperature difference between the system and the surroundings is zero

(b) The system is perfectly insulated

Thus for a gas undergoing an adiabatic process

Q = 0

U2 - U1 = mcv(T2 - T1)

W = (p1V1 - p2V2)/(n-1) = mR(T1 - T2)/(n-1)

But Q - W = U2 - U1 ( NFEE)

∴∴∴∴ 0 - mR(T1 - T2)/(n-1) = mcv(T2 -T1)

∴∴∴∴ R(T2 - T1)/(n-1) = cv(T2 - T1) ∴∴∴∴R/(n-1) = cv ∴∴∴∴R/cv = n-1

But R = cp - cv for a gas ∴∴∴∴(cp - cv)/cv = n-1 ∴∴∴∴cp/cv - 1 = n-1

∴∴∴∴ n = cp/cv

Thus an adiabatic process for a gas is a special case of a polytropic process in which the index of

the process "n" is equal to the ratio of the specific heats of the gas.

The ratio of the specific heats of the gas is written as γ (gamma), ie. γ = cp/cv

Hence the process law for a gas undergoing an adiabatic process is

pVγ = constant

i.e. p1V1γ = p2V2

γ where γ = cp/cv

Also W = (p1V1 - p2V2)/(γ - 1) or W = mR(T1 - T2)/(γ-1)

And T2/T1 = (p2/p1)(γ-1)/γ = (V1/V2)

(γ-1)

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p – V DIAGRAMS FOR NON-FLOW PROCESSES

AIR AS THE WORKING FLUID

Many thermodynamic systems use air as the working fluid. When this is the case, values of R,

the specific heats, etc. may be obtained from page 26 of the tables of Thermodynamic and

Transport Properties of Fluids by Rogers and Mayhew, and these are given below:

R kJ/kgK cp kJ/kgK cv kJ/kgK

0.287 1.005 0.718

p

V

p

V

p

V

2

1

1 2

1

2

Constant volume heating process Constant pressure expansion process

Constant temperature expansion processi.e. an isothermal expansion

V = Cp = C

pV = C

p

V

1

2

pVn = C

Polytropic expansion process

(sdr-thermo\p-V diagrams)


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EXERCISE - REVERSIBLE NON-FLOW PROCESSES – GASES (Thermodynamics\ Level 1\Non-flow Gas Exercise.doc)

1. A rigid container holds 1kg of air initially at 4.8bar and 150oC. The air is heated until its

temperature is 200oC. Determine (a) the final air pressure (b) the work done (c) the change in internal energy (d) the heat supplied. For air, R = 0.287 kJ/kgK, cv = 0.718 kJ/kgK

[Ans. 5.37bar; 0; 35.9kJ; 35.9kJ] 2 A mass of gas is heated at a constant pressure of 1.5bar in a closed system from an initial

volume of 0.25m3 to a final volume of 0.75m3. The initial temperature is 15oC. Determine (a) the mass of air (b) the final temperature (c) the work done (d) the internal energy change (e) the heat transfer.

Take R = 0.3kJ/kgK and cv = 0.73kJ/kg. [Ans 0.435kg; 864K; 75kJ; 183kJ; 258kJ] 3 A quantity of gas is compressed isothermally from an initial pressure of 1bar and an initial

volume of 1m3 through a volume ratio of 8. Determine (a) the work done (b) the change in internal energy (c) the heat transfer.

[Ans -208kJ, zero, -208kJ] 4 1m3 of air at 10bar and 150oC expands polytropically in a closed system to a final volume

of 6m3 according to the law PV1.2 = C. Determine (a) the mass of air (b) the final pressure (c) the final temperature (d) the work done (e) the internal energy change (f) the heat transfer.

Take R = 0.287kJ/kgK and Cv = 0.718kJ/kg. [Ans 8.25 kg, 1.162 bar, 295 K, 1500 kJ, -758 kJ, 742 kJ]


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5 1kg of gas expands adiabatically in a closed system from 227C and 7bar to a final pressure of 1.5bar. Determine (a) the final temperature (b) the initial volume (c) the final volume (d) the work done (e) the heat transfer (f) the change in internal energy.

Take Cp = 0.994kJ/kgK and Cv = 0.72kJ/kgK. [Ans 327 K, 0.1956 m3, 0.597 m3, 124.8 kJ, zero, -124.8 kJ] 6 A quantity of air, initially occupying a volume of 0.04m3 at a pressure of 15bar and a

temperature of 500oC, is contained in a closed cylinder fitted with a piston. The air is allowed to expand polytropically, displacing the piston, until its final

volume is 0.32m3, the law of the expansion process being pV1.25 = constant. Assuming the process to be reversible and treating the air as a perfect gas, calculate

for the air (a) the mass; (b) the final pressure; (c) the final temperature; (d) the change in internal energy; (e) the work done; (f) the heat transfer. For air: R = 287J/kgK and cv = 718J/kgK. [Ans 0.2705 kg, 1.115 bar, 459.6oK, -60.86 kJ, 97.28 kJ, 36.42 kJ] 7 A quantity of air is compressed polytropically in a closed piston-in-cylinder type

reciprocating compressor, the law of the process being pV1.3 = constant. At the beginning of the process, the air has a volume of 0.02m3, a pressure of 1.2 bar and

a temperature of 10oC. At the end of the process, the volume of air is 0.002m3. Assuming the process to be reversible, and treating the air as a perfect gas, calculate for

the air in the cylinder: (a) the mass (b) the final temperature (c) the final pressure (d) the work done (e) the change in internal energy (f) the heat transfer For air, R =287 J/kgK and cv = 718 J/kgK [Ans: 0.02955 kg, 564.7 K, 23.94 bar, -7964 J, 5977 J, -1987 J]


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8 Two points were chosen on the compression curve of an indicator diagram taken from a reciprocating air compressor. At the first point it was found that the pressure was 110kN/m2 and the volume was 0.027m3, whilst at the second point the pressure was 350kN/m2 and the volume was 0.01m3. The mass of air under compression was 0.027kg.

Given that the compression process was polytropic and followed the law PVn = constant, determine for the process between the two chosen points

(a) the polytropic index n (b) the initial and final temperatures (c) the work done on the air. Take R = 0.287kJ/kgK.

[Ans 1.165, 383.3K, 451.7K, -3.21kJ] 9 0.4kg of gas is compressed adiabatically in a closed system from 20C and 1bar to a final

pressure of 6bar. Determine (a) the ratio of the specific heats γ

(b) the gas constant R (c) the final temperature (d) the initial volume (e) the final volume (f) the work done (g) the heat transfer (h) the change in internal energy. Take cp = 1.008 kJ/kgK and cv = 0.72 kJ/kgK.

[Ans 1.4, 0.288 kJ/kgK, 488.9 K, 0.3375 m3, 0.09387 m3, -56.42 kJ, zero, 56.42 kJ] 10 A quantity of gas expands isothermally from an initial pressure of 5 bar and an initial

volume of 0.2m3 to a final volume of 0.9m3. Determine (a) the final pressure

(b) the work done (c) the change in internal energy (d) the heat transfer. [Ans 1.111 bar, 150.41 kJ, zero, -150.41 kJ] 11 The cylinder of a reciprocating compressor, initially of volume 0.1 m3, contains 0.25 kg

of nitrogen at a pressure of 2.5 bar. The gas is subjected to a reversible adiabatic compression process to a final volume of

0.04 m3. Assuming the nitrogen to be a perfect gas, calculate:- (a) Its initial temperature (b) The ratio of its specific heats (γ) (c) Its final pressure at the end of the compression stroke (d) Its final temperature (e) The work done in compressing the nitrogen (f) The change in internal energy of the nitrogen For nitrogen: R = 297 J/kgK and cv = 745 J/kgK. [Ans 336.7 K, 1.4, 9.02 bar, 485.8 K, -27.7 kJ, 27.7 kJ]


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12 Two points were chosen on the expansion curve of an indicator diagram taken from a piston engine. At the first point it was found that the pressure was 24 bar and the volume was 0.002 m3, whilst at the second point the pressure was 1.2 bar and the volume was 0.02 m3. The mass of gas in the cylinder was 0.03 kg. Given that the expansion procerss was polytropic and followed the law PVn = constant, determine for the process between the two chosen points

(a) the polytropic index n (b) the initial and final temperatures (c) the work done by the expanding gases. Take R = 0.287kJ/kgK.

[Ans 1.301, 557.5K, 278.7K, 7975J]


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STEADY-FLOW PROCESSES – GASES (Thermodynamics\ Level 1\ Steady-flow Gas.doc)

OPEN SYSTEMS

Flow processes take place in open systems. An example is a gas turbine engine

Alternatively, the compressor on its own, or the combustion chamber, or the turbine may be considered separately as open systems.

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THE GAS LAWS

The gas laws given below apply to any process where the working fluid is a gas.

PV/T = mR = Constant pv/T = R for 1 kg R = cp - cv p1V1/T1 = p2V2/T2

U2 - U1 = mcv(T2 - T1) u2 - u1 = cv(T2 - T1) H2 - H1 = mcp(T2 - T1) h2 - h1 = cp(T2 - T1)

Fluid in Fluid out



Real boundary with gaps

Turbine rotor(thermo\turbine1.sdr)

OPEN SYSTEM

TURBINE


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THE STEADY FLOW ENERGY EQUATION

Q – W = mg(Z2 –Z1) + ½m(C22 - C1

2) + (H2 – H1)

Or

q – w = g(Z2 –Z1) + ½(C22 - C1

2) + (h2 – h1) for a mass of 1kg

H2 - H1 = mcp(T2 - T1) for all gas processes

Or

h2 - h1 = cp(T2 - T1) for a mass of 1kg

UNITS Always work in Joules throughout when using the SFEE, as the potential energy and kinetic energy terms will naturally come out in Joules when using kilograms, metres and seconds.

---------------------------------------------------------------------------- THE CONTINUITY EQUATION

ρρρρ1A1C1 = ρρρρ2A2C2 or A1C1/v1 = A2C2/v2

ACm ρρρρ====& = AC/v v = 1 / ρρρρ

Where v = specific volume and ρρρρ = density

Units: &m = kg/s ρρρρ = kg/m3 v = m3 / kg A = m2 C = m/s

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SPECIFIC VOLUME OF A GAS v m 3/kg

Specific volume is the volume occupied by 1 kg of gas i.e. v = V/m

pV = mRT for a gas, so v = V / m = RT / p

v = RT / p for a gas

Note that specific volume v is the inverse of density ρ, i.e.

v = 1 / ρρρρ

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POWER

Power = the rate of doing work = the work done per second

∴∴∴∴ Power = work done by each kg of gas x the number of kg of gas flowing per second

∴∴∴∴ Power = work done by each kg of gas x the mass flow rate of gas

∴∴∴∴ Power = work done per kg of gas x m&

∴∴∴∴ work done per kg of gas = power / m&

Units

Power Work done per kg of gas Mass flow rate m&

W or J/s J/kg Kg/s


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EXERCISE - STEADY FLOW PROCESSES – GASES (Thermodynamics\ Level 1\ Steady-flow Gas Exercise)

Note. Unless otherwise stated, always do the initial calculations taking the mass

of gas as 1 kg. Then take the actual mass flow of gas into account at the end of the calculation.

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1 A steady flow of air enters a horizontal heater at 9o C and leaves at 32o C. The mass flow of air through the heater is 0.6 kg/s.

The air pressure at the exit from the heater is 1.2 bar Ignoring any changes in kinetic energy, calculate (a) The change in specific enthalpy of the air i.e. the enthalpy change per kg (b) The heat transfer per kg of air (c) The heater power i.e. the heat transfer per second (d) The specific volume of the air as it leaves the heater (e) The cross-sectional area of the heater exit duct if the exit velocity is 20m/s. Take R = 287 J/kgK and cp = 1005 J/kgK [Ans. 23.115 kJ/kg, 23.115 kJ/kg, 13.869 kW, 0.7295 m3/kg, 0.02188 m2] 2 A steady flow of air enters a horizontal cooler at 95o C and 120 m/s and it leaves at

20o C and 10 m/s. The air pressure at the entry to the cooler is 2.5 bar. The mass flow of air through the cooler is 1.8 kg/s. Calculate

(a) The change in specific enthalpy of the air between entry and exit (b) The change in kinetic energy per kg of air between entry and exit (c) The heat transfer per kg of air (d) The heat transfer rate i.e. the heat transfer per second (e) The specific volume of the air at entry (f) The cross-sectional area of the cooler entry duct. Take R = 0.287 kJ/kgK and cp = 1.005 kJ/kgK [Ans. –75.375 kJ/kg, -7.15 kJ/kg, -82.525 kJ/kg, -148.545 kJ/s or kW, 0.4225 m3/kg, 6337 mm2] 3 Air enters a horizontal nozzle at a pressure of 2 bar, a temperature of 93o C and a

velocity of 50 m/s. The air expands adiabatically in the nozzle, leaving the nozzle with a temperature of 27o C and a pressure of 1 bar. The air mass flow rate is 3.8 kg/s. Calculate

(a) The exit velocity (b) The specific volume of the air at entry (c) The cross-sectional area of the nozzle entry duct. Take R = 0.287 kJ/kgK and cp = 1.005 kJ/kgK [Ans. 367.64 m/s, 0.52521 m3/kg, 0.03992 m2]


17

4 A steady flow of gas enters the horizontal turbine of a jet engine at 900o C and 20 m/s, and leaves at 500o C and 180 m/s. A heat loss of 10 kJ/kg of gas occurs during its passage through the turbine and the mass flow rate is 5 kg/s. The pressure of the gas at the turbine exit is 100 kN/m2. Calculate

(a) The change in specific enthalpy of the gas between entry and exit (b) The change in kinetic energy per kg of gas between entry and exit (c) The work done per kg of gas (d) The power output of the turbine (e) The specific volume of the gas at the turbine exit (f) The cross-sectional flow area of the turbine exhaust duct. Take cp = 1.080 kJ/kgK and R = 0.295 kJ/kgK [Ans. –432 kJ/kg, 16 kJ/kg, 406 kJ/kg, 2.03 MW, 2.28035 m3/kg, 0.06334 m2] 5 A steady stream of air enters a horizontal compressor at the rate of 0.02 kg/s. The air

temperature is 10o C at entry and 300o C at exit. The velocity of the air is 30 m/s at entry and 6 m/s at exit. During its passage through the compressor, the air experiences a heat loss of 105 kJ/kg of air. Calculate

(a) The change in kinetic energy per kg of air between entry and exit (b) The change in specific enthalpy of the air between entry and exit (c) The work done per kg of air (d) The power required to drive the compressor (e) The flow area of the entry pipe if the entry pressure is 1 bar Take R = 0.287 kJ/kgK and cp = 1.005 kJ/kgK for the air. [Ans. –0.432 kJ/kg, 291.45 kJ/kg, -396.018 kJ/kg, 7.92036 kW, 541.47 mm2] 6 A gas flows steadily into a horizontal cooler at 275o C and 4.6 bar through a circular

duct of 200 mm diameter. The gas leaves the cooler at 35o C and 15 m/s. The mass flow rate of the gas through the cooler is 1.9 kg/s. Calculate

(a) The flow area of the entry duct and the specific volume of the gas at entry (b) The entry velocity and the change in kinetic energy per kg of gas (c) The change in specific enthalpy between entry and exit (d) The heat transfer per kg of gas (e) The heat transfer rate i.e. the heat transfer per second Take R = 0.290 kJ/kgK and cp = 1.010 kJ/kgK [Ans. 0.0314159 m2, 0.345478 m3/kg, 20.894 m/s, -0.10578 kJ/kg, -242.4 kJ/kg, -242.50578 kJ/kg, -460.761 kJ/s or kW]


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7 A horizontal axial compressor provides a steady flow of air. The air enters the compressor at a velocity of 5 m/s, a temperature of 15o C and a pressure of 1 bar. The air leaves the compressor at a velocity of 85 m/s and a temperature of 115o C. The outer surfaces of the compressor are exposed to the atmosphere and as a result, there is a heat loss of 6 kJ/kg of air as it passes through the compressor. The mass flow rate is 4 kg/s.

Calculate for the air flowing through the compressor: (a) The change in enthalpy per kg of air (b) The change in kinetic energy per kg of air (c) The work done per kg of air (d) The power required to drive the compressor (e) The air density at entry (f) The cross-sectional flow area of the entry pipe. Take R = 0.287 kJ/kgK and cp = 1.005 kJ/kgK [Ans. 100.5 kJ/kg, 3.6 kJ/kg, -110.1 kJ/kg, 440.4 kW, 1.2098 kg/m3, 0.6612 m2] 8 Air flows steadily through a horizontal nozzle that is designed to produce a stream of

high velocity air at its exit. The air enters the nozzle at a pressure of 3.2 bar, a temperature of 100o C and with a velocity of 6m/s. The air expands adiabatically as it flows through the nozzle and leaves the nozzle with a pressure of 2 bar. The air mass flow rate is 0.8 kg/s. Calculate

(a) The temperature of the air as it leaves the nozzle (b) The enthalpy change per kg of air (c) The kinetic energy per kg of air at entry to the nozzle (d) The kinetic energy per kg of air as it leaves the nozzle (e) The velocity of the air as it leaves the nozzle (f) The density of the air at the nozzle exit (g) The required flow area at the nozzle exit Take R = 287 J/kgK, cp = 1005 J/kgK and γ = 1.4

Note that T2 / T1 = (p2 / p1)(γγγγ-1)/γγγγ for a gas undergoing an adiabatic process

[Ans. 326.1o K, -47.106 kJ/kg, 18 J/kg, 47.124 kJ/kg, 307 m/s, 2.137 kg/m3 1220 mm2]


19

9 A steady stream of air enters a horizontal compressor at 10o C with a velocity of 50 m/s.

The air leaves the compressor at 300o C with a velocity of 10 m/s. During its passage through the compressor, the air experiences a heat loss of 20 kJ/kg of air. The air pressure at entry is 1 bar and the mass flow rate is 0.2 kg/s. Calculate:

(a) The change in kinetic energy per kg of air (b) The change in enthalpy per kg of air (c) The work done per kg of air (d) The power required to drive the compressor (e) The air density at entry (f) The cross-sectional flow area of the entry pipe. Take R = 287 J/kgK and cp = 1000 J/kgK. [Ans. –1.2 kJ/kg, 290 kJ/kg, -308.8 kJ/kg, 61.76 kW, 1.2312 kg/m3, 3248 mm2] 10 Air flows steadily through a horizontal heater. The air enters the heater at a temperature

of 20o C, and with negligible velocity. The air leaves the heater at a pressure of 1.4 bar, a temperature of 420o C, and a velocity of 200 m/s. The air mass flow rate is 2.4 kg/s.

Determine for the air (a) The change in kinetic energy per kg (b) The change in enthalpy per kg (c) The heat transferred per kg (d) The density of the air at exit (e) The flow area of the exit pipe. Take R = 287 J/kgK and cp = 1005 J/kgK [Ans. 20 kJ/kg, 402 kJ/kg, 422 kJ/kg, 0.7039 kg/m3, 0.01705 m2] 11 A steady flow of air enters a compressor with a velocity of 60 m/s, a temperature of 20o C

and a pressure of 1 bar, through a pipe that has a cross-sectional area of 0.5 m2. The air leaves the compressor with a velocity of 70 m/s and a pressure of 2 bar through a pipe that has a cross-sectional area of 0.4 m2. There is a heat loss of 1350 J per kg of air as it passes through the compressor. Calculate for the air flowing through the compressor:

(a) The volumetric flow rate at entry in m3/s (b) The mass flow rate in kg/s (c) The volumetric flow rate at exit in m3/s (d) The air temperature at exit (e) The change in enthalpy per kg of air (f) The change in kinetic energy per kg of air (g) The work done per kg of air (h) The power required to drive the compressor. Take R = 287 J/kgK and cp = 1005 J/kgK [Ans. 30 m3/s, 35.68 kg/s, 28 m3/s, 546.9oK, 255.2 kJ/kg, 0.65 kJ/kg, -257.2 kJ/kg,

9.18MW]


20

12 A steady flow of gas enters the turbine of a jet engine at 900o C and 20 m/s, and leaves at

500o C and 180 m/s. A heat loss of 10 kJ/kg of gas occurs during its passage through the turbine and the mass flow rate is 5 kg/s.

Determine the power output of the turbine and also the exhaust duct flow area given that the exhaust pressure is 100 kN/m2.

Take Cp = 1 kJ/kgK and R = 0.290 kJ/kgK. [Ans. 1.87 MW; 0.0622 m2] 13 Air enters a horizontal nozzle at a pressure of 1.6 bar, a temperature of 150o C, and a

velocity of 150 m/s. The air expands adiabatically and reversibly (i.e. without friction) in the nozzle, leaving the nozzle with an exit pressure of 1 bar. Calculate

(a) The air temperature at the nozzle exit (b) The air velocity at the nozzle exit. Take R = 287 J/kgK, cp = 1005 J/kgK and γ = 1.4

Note that T2 / T1 = (p2 / p1)(γγγγ-1)/γγγγ for a gas undergoing an adiabatic process

[Ans. 96.8o C, 359.8 m/s] 14 A perfect gas flows steadily through a cooler. The entry temperature and velocity are

315o C and 150 m/s respectively. The gas leaves the cooler through a duct of cross-sectional area 0.2 m2 at a pressure of 250 kN/m2 and at a temperature of 37o C. The mass flow rate of the gas is 13.5 kg/s. Calculate the heat transfer rate.

Take R = 0.297 kJ/kgK and Cp = 1.035 kJ/kgK. [Ans. -4.04 MJ/s or -4.04 MW] 15 A steady stream of air enters a compressor at the rate of 0.8kg/min. The air temperature

is 15oC at inlet and 30oC at exit. The compressor is cooled by 4 kg/min of cooling water, the temperature rise of which is 20 degC. The velocity of the air is 20m/s at entry and 5m/s at exit. Determine the power required to drive the compressor, stating any assumptions made, and also the cross-sectional area of the entry pipe if the inlet pressure is 1bar.

Take the specific heat of water = 4.187 kJ/kgK Take R = 0.287 kJ/kgK and cp = 1.005 kJ/kgK for the air [Ans. 5.79 kW, 0.00055 m2]


21

16 A centrifugal compressor takes in 0.5 kg/s of air at 1.013 bar and 15o C and compresses the air to a final pressure of 1.9 bar according to the law pV1.6 = C. Assuming that any heat losses through the casing and changes in potential energy and kinetic energy are all negligible, calculate

(a) The temperature of the air leaving the compressor 91.6 o C (b) The power required to drive the compressor 38.49 kW Take R = 0.287 kJ/kgK and cp = 1.005 kJ/kgK Note T2 / T1 = (p2 / p1)

(n-1)/n for a gas undergoing a process to the law pVn = C 17 A horizontal "hot air blower" consists of a fan, an electrical heating element and a nozzle

as shown.

The power input to the fan is 65J/s and the power input to the heating element is 1155J/s.

During its passage through the blower, the air experiences a heat loss of 15J/s. A steady flow of air enters the fan at a pressure of 1bar, a temperature of 24oC, a

velocity of 10m/s, and a volumetric flow rate of 0.02m3/second. After passing over the heating element, the air leaves the nozzle at a pressure of 1.1bar

and a temperature of 75oC. Treating the complete arrangement as a steady flow system, determine: (a) The mass flow rate of air entering the fan in kg per second 0.02346kg/s (b) The change in enthalpy of the air occurring per second between entering the fan and leaving the nozzle 1202.4J/s (c) The kinetic energy of the air entering the fan per second 1.2J/s (d) The kinetic energy of the air leaving the nozzle per second 3.8J/s (e) The velocity of the air as it leaves the nozzle 18m/s (f) The density of the air as it leaves the nozzle 1.101kg/m3 (g) The flow area at the nozzle exit. 1184mm2 For air, take R = 287 J/kgoK and cp = 1005 J/kgoK

Fan Heater Nozzle

1 2

(thermo\ blower.sdr)

Cool air Hot air


22

18 A horizontal "hot air blower" consists of a fan, an electrical heating element and a nozzle. The power input to the fan is 50J/s and the power input to the heating element is 1.44kJ/s. The air enters the fan with negligible velocity at a pressure of 1 bar and a temperature of

20o C, the volumetric flow rate being 0.5 m3/min. After passing over the heating element, the air leaves the nozzle with a velocity of 25 m/s and a pressure of 1.1bar.

Treating the complete arrangement as a steady flow system and ignoring any heat loss from the system, determine:

(a) The mass flow rate of air entering the fan in kg per second 0.00991kg/s (b) The kinetic energy of the air leaving the nozzle per second 3.1J/s (c) The change in enthalpy of the air occurring per second between entering the fan and leaving the nozzle 1486.9J/s (d) The temperature of the air as it leaves the nozzle. 169.3o C (e) The density of the air as it leaves the nozzle 0.8666kg/m3 (f) The flow area at the nozzle exit. 457.4x10-6m2 Take R = 0.287 kJ/kgK and cp = 1.005 kJ/kgK 19 A steady stream of air enters a horizontal compressor at the rate of 0.02 kg/s. The air

temperature is 10o C at entry and 300o C at exit. The velocity of the air is 30 m/s at entry and 6 m/s at exit. During its passage through the compressor, the air experiences a heat loss of 105 kJ/kg of air. Calculate:

(a) The change in kinetic energy per kg of air –0.432 kJ/kg (b) The change in enthalpy per kg of air 291.45 kJ/kg (c) The work done per kg of air -396 kJ/kg (d) The power required to drive the compressor 7.92 kW (e) The volume of air entering the compressor per second 0.01624 m3/kg (f) The flow area of the entry pipe if the entry pressure is 1 bar 541.5 mm2 Take R = 0.287 kJ/kgK and cp = 1.005 kJ/kgK ___________________________________________________________________________

Fan Heater Nozzle

1 2


Cool air Hot air


23

20 The diagram below shows a “hot air blower” consisting of a fan, an electrical heating element and a nozzle. The flow of air through the blower is horizontal

The power input to the fan is 80J/s and the power input to the heating element is

1500J/s. During its passage through the blower, the air experiences a heat loss of 10J/s. A steady flow of air enters the fan at a pressure of 1bar, a temperature of 20oC, a

velocity of 9m/s, and a volumetric flow rate of 0.015m3/second.After passing over the heating element, the air leaves the nozzle at a pressure of 1.2bar and a temperature of 107oC.

Treating the complete arrangement as a steady flow system, determine: (a) The mass flow rate of air entering the fan in kg per second 0.01784kg/s (b) The change in enthalpy of the air occurring per second between entering the fan and leaving the nozzle 1559.7J/s (c) The kinetic energy of the air entering the fan per second 0.72J/s (d) The kinetic energy of the air leaving the nozzle per second 11.1J/s (e) The velocity of the air as it leaves the nozzle 35.23m/s (f) The density of the air as it leaves the nozzle 1.1kg/m3 (g) The flow area at the nozzle exit. 460mm2 For air, take R = 287 J/kgoK and cp = 1005 J/kgoK ___________________________________________________________________________

Fan Heater Nozzle

1 2


Cool air Hot air


24

STEAM TABLES (Thermodynamics\ Level 1\ Steam-tables.doc)

CONSTANT PRESSURE HEATING OF WATER TO PRODUCE STEAM

• Water changes into steam at the saturation temperature (boiling point)

• The saturation temperature increases with increasing pressure

• Water at its boiling or saturation temperature is known as saturated water

• The temperature remains constant while water is being changed into steam

• Steam at its saturation temperature is known as dry saturated steam

• When water is changing to steam, a water/steam mixture known as wet steam exists

• Steam above its saturation temperature is known as superheated steam

• Steam does not obey the Gas Laws

• Never use the Gas Laws for steam

• Always use Steam Tables for finding the properties of water and steam.

• The recommended tables are:

• Thermodynamic & Transport Properties of Fluids (SI units) by Rogers & Mayhew

Ice

Water

Superheated steam

(thermo\water-steam.sdr)

Temperature

Heat supplied

Saturation temperatureor boiling point

Fusion temperatureor freezing point

Wet steam

Dry saturated steam

Saturated water


25

Thermodynamic & Transport Properties of Fluids (SI units)

by Rogers & Mayhew

These tables use the following notation and units:

Property Symbol Units Pressure p bar

Temperature t oC

Specific volume v m3/kg

Specific internal energy u kJ/kg

Specific enthalpy h kJ/kg

Specific entropy s kJ/kg oK

SUBSCRIPTS USED IN THE TABLES

f refers to water at its boiling point or saturation temperature

g refers to dry steam that is still at the saturation temperature (dry saturated steam)

fg refers to the change from boiling water to dry saturated steam at constant pressure

s refers to the saturation temperature or pressure

---------------------------------------------------------------------------------

PROPERTIES OF WATER BELOW ITS BOILING POINT A liquid that is below its boiling point or saturation temperature is known as a compressed

liquid.

Water is highly incompressible so its properties are little affected by changes in pressure.

For this reason the actual temperature of the water is taken as the boiling

or saturation temperature and the actual pressure is ignored. So:

v = vf (page 10) u = uf (pages 3 – 5) h = hf (pages 3 – 5)

Do not forget to insert two zeros after the decimal point in each case for vf.

For example, at 25o C, vf = 0.0010030 m3 / kg and not 0.10030 m3 / kg

-----------------------------------------------------------------------------


26

PROPERTIES OF WATER AT ITS BOILING POINT

This is water at its saturation temperature. It is known as “saturated water”

Subscript “f ” applies. So:

v = vf (page 10) u = uf (pages 3 to 5) h = hf (pages 3 to 5)



-----------------------------------------------------------------

PROPERTIES OF DRY SATURATED STEAM

Dry saturated steam is steam that has no particles of water contained in it but that is still at

the saturation temperature (boiling point) at which it was formed.

Subscript “g” applies. So:

v = vg (pages 3 to 5) u = ug (pages 3 to 5) h = hg (pages 3 to 5)

-------------------------------------------------------------------------------

PROPERTIES OF WET STEAM

Wet steam is a mixture of boiling (saturated) water and dry saturated steam that exists during

the change of phase from boiling water to dry saturated steam.

The proportion of dry steam that is present in a wet steam mixture is known as the

Dryness fraction x

mixturesteamwettheofmasstotalthemixturesteamwettheinpresentsteamdryofmassthe

xfractionDryness ====∴∴∴∴

Thus in 1 kg of wet steam with dryness fraction x

Mass of dry steam present = x kg

Mass of water present = (1 – x) kg


27

Therefore

v = xvg + (1 – x)vf u = xug + (1 – x)uf h = xhg + (1 – x)hf

Expanding the expression for h gives

h = xhg + hf – xhf or h = hf + x(hg – hf) or

h = hf + xhfg

Values of uf, hf, ug, hg and hfg are given on pages 3 – 5 of the tables.

Values of vf are given on page 10 of the tables.



----------------------------------------------------------------------------

APPROXIMATE EXPRESSION FOR SPECIFIC VOLUME OF WET

STEAM

The volume of water present in wet steam is normally negligible compared with the volume

of dry steam that is present. The volume of the water present may then be ignored and thus

The approximate specific volume of wet steam v = xvg

PROPERTIES OF SUPERHEATED STEAM

This is steam that has been heated to temperatures in excess of the saturation temperature

corresponding to the steam pressure.

v, u and h are given in the tables at various pressures and temperatures on pages 6 – 8.

To determine values of v, u and h at temperatures that are at different pressures and

temperatures to those given in the tables, linear interpolation may be used.

-------------------------------------------------------------------------------


28

STEAM TABLES - EXERCISE

Use steam tables to determine the specific enthalpy (h), the specific internal energy (u) and

the specific volume (v) for the following cases.

Remember that the pressures in the tables are expressed in bars.

Note 100 kN/m2 = 105 N/m2 = 1 bar

Also 1 MN/m2 = 106 N/m2 = 10bar

Answers in kJ / kg or m3 / kg

1 Water at 88o C and 1 bar [369, 369, 0.001034]

2 Water at it saturation temperature at 2.7 bar [546, 546, 0.00107]

3 Dry saturated steam at 5 bar [2749, 2562, 0.3748]

4 Wet steam with dryness faction 0.9 at 10 bar [2576.5, 2401.8, 0.17507]

5 Superheated steam at 6 bar and 250o C [2958, 2722, 0.394]

6 Superheated steam at 6 bar and 260o C [2978.8, 2737.8, 0.4021]

7 Water at normal atmospheric pressure and 80o C [334.9, 334.9, 0.001029]

8 Water at its saturation temperature at 200 kN/m2 [505, 505, 0.00106]

9 Saturated steam at 400 kN/m2 [2739, 2554, 0.4623]

10 Wet steam with dryness fraction 0.8 at 2 MN/m2 [2421, 2261.4, 0.07989]

11 Steam at 1 MN/m2 and 225o C [2886.5, 2667, 0.21945]

12 Water at 165o C and 600 kN/m2 [697, 696, 0.001108]

13 Boiling water at 2.6 MN/m2 [972, 969, 0.0012014]

14 Saturated steam at 39 bar [2801.5, 2602, 0.051115]

15 Wet steam with dryness fraction 0.92 at 16.5 bar

Use the approximate method to determine v [2640.18, 2457.86, 0.110584]

16 Steam at 550 kN/m2 and 320o C [3104.9, 2834.3, 0.49598]


29

NON-FLOW PROCESSES – STEAM

(Thermodynamics\ Level 1\ Non-flow steam.doc)

Simple Steam Engine

CLOSED SYSTEMS Non-flow processes take place in closed systems

-------------------------------------------------------------------------------- THE NON-FLOW ENERGY EQUATION – applies to all non-flow processes

Q - W = (U2 - U1) or q - w = (u2 - u1) per kg

(U2 - U1) is the change in internal energy of the system

Piston



CLOSED SYSTEM

CYLINDER & PISTON

Real continuous boundarywhen valves are closed

(thermo\cylinder.sdr)


30

STEAM TABLES

• Steam does not obey the Gas Laws

• Do not use the Gas Laws for steam

• Always use Steam Tables for finding values of internal energy, specific

volume, enthalpy, etc. for water and steam.

• The recommended tables are:

• Thermodynamic & Transport Properties of Fluids (SI units) by Rogers

& Mayhew

---------------------------------------------------------------------------- WORK DONE IN NON-FLOW PROCESSES W

Process Process law Work done W

Constant volume V1 = V2 =constant Zero

Constant pressure p1 = p2 = constant p(V2 – V1)

Hyperbolic p1V1 = p2V2 = constant p1V1ln(V 2 / V1)

Polytropic p1V1n = p2V2

n = constant (p1V1 – p2V2) / (n – 1)

• A hyperbolic process following the law pV=constant is NOT an

isothermal process (constant temperature process) when the working fluid

is steam, (although it is when the working fluid is a gas)

• When steam undergoes an adiabatic process (zero heat transfer, Q = 0), it

does NOT follow the process law pVγγγγ = constant (this only applies to

gases undergoing an adiabatic process)


31

EXERCISE – NON-FLOW PROCESSES - STEAM 1 Steam at a pressure of 9 bar and 0.9 dry occupies a volume of 0.42 m3 in the cylinder of

a steam engine. It is expanded according to the law PV1.35 = C to a pressure of 3 bar. Determine: (a) The mass of steam present (b) The final volume (c) The final dryness fraction (d) The external work done (e) The change in internal energy (f) The heat transfer. [Ans 2.17 kg, 0.948 m3, 0.72, 267.7 kJ, -884.5 kJ, -616.8 kJ] 2 At a certain point during the expansion process in a steam engine cylinder, the steam

pressure was 1.1 MN/m2 and the dryness 0.85. At the end of the expansion process the pressure was 0.28 MN/m2 and the dryness was 0.80. Assuming the expansion obeys the law PVn = C, determine

(a) The value of the index n (b) The work done per kg (c) The internal energy change per kg (d) The heat transfer per kg. [Ans 1.1112, 191 kJ/kg, -172 kJ/kg, 19 kJ/kg] 3 The cylinder of a steam engine contains 0.2 kg of steam at 1.3 MN/m2 and 0.9 dry. If

the steam is expanded until the pressure falls to 0.6 MN/m2, the law of expansion being PV = constant, determine

(a) The final dryness fraction (b) The temperature at the end of the expansion (c) The internal energy change (d) The work done (e) The heat transfer [Ans 0.935, 158.8oC, 6.38 Kj, 27.38 kJ, 33.76 kJ] 4 2m3 of steam at 2 bar and 0.9 dry is compressed in a closed system according to the law

PVn = costant until the steam is at 10 bar and 200oC. Determine (a) The mass of steam present in the cylinder (b) The value of the index n (c) The work done (d) The internal energy change (e) The heat transfer. [Ans 2.51 kg, 1.19, -616.9 kJ, 741.4 kJ, 124.5 kJ]


32

5 Steam, initially 0.85 dry at 500 kN/m2 and occupying a volume of 0.5 m3, expands in the cylinder of an engine according to the law PV = constant until the final pressure at the end of the expansion process is 100 kN/m2. Determine

(a) The mass of steam (b) The final volume (c) The final dryness fraction (d) The work done (e) The change in internal energy (f) The heat transfer [Ans 1.569 kg, 2.5 m3, 0.941, 402.4 kJ, 170.5 kJ, 572.9 kJ] 6 1 kg of steam 0.9 dry at 12 bar is expanded in a non-flow process until its final pressure

is 1.2 bar according the law PV1.15 = C. Determine (a) The final dryness fraction (b) The work done (c) The internal energy change (d) The heat transfer [Ans 0.762, 305.1 kJ, -390 kJ, -84.9 kJ] 7 A rigid vessel has a volume of 1m3 and contains steam at 2 MN/m2 and 400oC. The

vessel is cooled until the steam is just dry saturated. Determine (a) The mass of steam present (b) The final pressure (c) The heat transfer for the process [Ans 6.618 kg, 13 bar, -2356 kJ] 8 1kg of wet steam having a dryness fraction of 0.8 and a pressure of 10bar expands in the

closed cylinder of an engine, the relationship between the pressure and the volume during the process being given by the law pV = constant. At the end of the expansion process the steam pressure is 1bar and the steam is still wet. Determine for the steam:

(a) Its initial specific volume (b) Its final specific volume (c) Its final dryness fraction (d) Its initial specific internal energy (e) Its final specific internal energy (f) The work done (g) The heat transfer Note. The approximate formula for the specific volume of wet steam may be used in

your solution, i.e. v = xvg [Ans 0.15552m3/kg, 1.5552m3/kg, 0.9181, 2219.6kJ/kg, 2334.9kJ/kg, 358.1kJ, 473.4kJ]


33

9 1kg of superheated steam, initially at a pressure of 10bar and a temperature of 200oC, expands polytropically in the closed cylinder of an engine, the relationship between pressure and volume during the process being given by the law pV1.2 = constant. At the end of the expansion process the steam pressure is 2bar and the steam is wet.

Determine for the steam: (a) Its initial specific volume (b) Its final specific volume (c) Its final dryness fraction (d) Its initial specific internal energy (e) Its final specific internal energy (f) The work done (g) The heat transfer Note. The approximate formula for the specific volume of wet steam may be used in

your solution, i.e. v = xvg [Ans 0.2061m3/kg, 0.788m3/kg, 0.89, 2623 kJ/kg, 2307 kJ/kg, 242.5 kJ, -73.5 kJ] 10 The cylinder of a steam engine contains 1kg of steam at an initial pressure of 15bar and a

dryness fraction of 0.9. The steam then expands displacing the piston down the bore of the cylinder until the steam pressure has dropped to 3bar, the law of the expansion being pV = constant.

Determine for the steam: (a) Its initial specific volume (b) Its final specific volume (c) Its final dryness fraction (d) Its change in internal energy (e) The work done (f) The heat transfer Note. The approximate formula for the specific volume of wet steam may be used in

your solution, i.e. v = xvg [Ans 0.11853m3/kg, 0.59265m3/kg, 0.9785, 81.56 kJ, 286.2 kJ, 367.7 kJ] 11 A rigid vessel has a volume of 1m3 and contains superheated steam at 20bar and 400oC.

The vessel is then cooled with its volume remaining constant, until the steam pressure is 12bar, the steam being wet in its final condition. Determine:

(a) The initial specific volume of the steam (b) The mass of steam present (c) The dryness fraction of the steam after cooling (d) The initial specific internal energy of the steam (e) The final specific internal energy of the steam (f) The actual quantity of heat transferred during the process Note. The approximate formula for the specific volume of wet steam may be used in

your solution, i.e. v = xvg [Ans 0.1511m3/kg, 6.618kg, 0.926, 2946 kJ/kg, 2455 kJ/kg, -3249 kJ]


34

12 1 kg of ammonia is cooled in a closed system under constant pressure conditions from 90oC and 290.8 kN/m2 until it is 0.9 dry. Using page 13 of the tables, determine

(a) The enthalpy change (b) The heat transfer (c) The final volume if the volume of liquid present is ignored [Ans -362.1 Kj, -362.1 kJ, 0.37665 m3] 13 A rigid vessel having a volume of 0.1 m3 is filled with ammonia vapour and sealed. Its

temperature is then lowered to –12oC, and at this temperature the ammonia vapour has a dryness fraction of 0.52. The container is then allowed to warm up slowly until the ammonia vapour becomes just dry saturated. Using page 13 of the tables determine

(a) The temperature at which this occurs (b) The heat transfer that must have occurred during the warming process Neglect the volume of liquid present in the container [Ans 6oC, 248 kJ]

______________________________________________


35

STEADY FLOW PROCESSES – STEAM (Thermodynamics\ Level 1\ Steady-flow Steam.doc)

Steam Turbine Rotor

OPEN SYSTEMS

Flow processes take place in open systems. An example is a steam turbine

Fluid in Fluid out



Real boundary with gaps

Turbine rotor(thermo\turbine1.sdr)

OPEN SYSTEM

TURBINE


36

THE STEADY FLOW ENERGY EQUATION

Q – W = ∆∆∆∆PE + ∆∆∆∆KE + ∆∆∆∆H

Where ∆∆∆∆PE = the change in potential energy = mg(Z2 –Z1) ∆∆∆∆KE = the change in kinetic energy = ½m(C2

2 - C12)

∆∆∆∆H = the change in enthalpy = (H2 – H1)

So

Q – W = mg(Z2 –Z1) + ½m(C22 - C1

2) + (H2 – H1)

Or

q – w = g(Z2 –Z1) + ½(C22 - C1

2) + (h2 – h1) for a mass of 1kg

UNITS Always work in Joules throughout when using the SFEE, as the potential energy and kinetic energy terms will naturally come out in Joules when using kilograms, metres and seconds.

----------------------------------------------------------------------------


37

THE CONTINUITY EQUATION

ρρρρ1A1C1 = ρρρρ2A2C2 or A1C1/v1 = A2C2/v2

ACm ρρρρ====& = AC/v v = 1 / ρρρρ

Where v = specific volume and ρρρρ = density

Units: &m = kg/s ρρρρ = kg/m3 v = m3 / kg A = m2 C = m/s

-------------------------------------------------------------------------------

DETERMINATION OF PROPERTIES ( u, h and v etc ) For steam, (and other vapours such as ammonia etc) TABLES must be used to determine the values of specific enthalpy (h), specific volume (v) and any other property values that may be required. The Gas Laws must not be used as these do not apply to steam.

-----------------------------------------------------------------------------------


38

EXERCISE - STEADY FLOW PROCESSES - STEAM 1 Superheated steam enters a horizontal turbine at a pressure of 20 bar, a temperature of

250oC, and a velocity of 60 m/s. The steam leaves the turbine at a pressure of 0.3 bar, a dryness fraction of 0.95, and a velocity of 300 m/s. A heat loss of 15 kJ/kg of steam occurs during its passage through the turbine. Determine for the steam flowing through the turbine

(a) The change in kinetic energy per kg of steam (b) The enthalpy per kg of steam at entry (c) The enthalpy per kg of steam at exit (d) The work done by each kg of steam (e) The total power produced if the mass flow rate is 5kg/s

(f) The entry duct flow area if the mass flow rate is 5kg/s [Ans 43.2kJ/kg, 2904kJ/kg, 2508.2kJ/kg, 337.6kJ/kg, 1.688MW, 9292mm2]

2 Superheated steam enters a horizontal turbine at a pressure of 50bar, a temperature of

400oC and with a velocity of 327m/s. The steam exits the turbine at a pressure of 0.2bar, a dryness fraction of 0.9, and with a

velocity of 150m/s. A heat loss of 15kJ/kg of steam occurs during its passage through the turbine. Determine for the steam flowing through the turbine: (a) The change in kinetic energy per kg of steam

(b) The specific enthalpy at entry (c) The specific enthalpy at exit (d) The work done by each kg of steam, stating any assumptions made (e) The total power produced if the mass flow rate is 400kg/s (f) The cross sectional area of the entry duct if the mass flow rate is 400kg/s

[Ans –42.2kJ/kg, 3196kJ/kg, 2373kJ/kg, 850kJ/kg, 340MW, 0.0707m2]


39

3 In a simple steam plant, a steady supply of saturated water (i.e. water at its saturation temperature) enters a boiler at a pressure of 10bar, and is converted into superheated steam at the same pressure.

The steam leaves the boiler at 10bar and 300oC and enters a turbine at the same pressure and temperature. The steam flows steadily through the turbine without heat loss, and leaves the turbine as wet steam with a dryness fraction of 0.9 and a pressure of 1bar.

Changes in potential energy and kinetic energy throughout the system are negligible. Determine (a) the specific enthalpy of the water at entry to the boiler (b) the specific enthalpy of the steam at the boiler exit (c) the specific enthalpy of the steam at the turbine exit (d) the heat transfer in the boiler per kg of steam (e) the work output from the turbine per kg of steam (f) the thermal efficiency of the combined boiler and turbine (g) the required flow area at entry to the turbine if the steam velocity at that point is 80m/s when the mass flow rate is 1.5kg/s

[Ans 763kJ/kg, 3052kJ/kg, 2449.2kJ/kg, 2289kJ/kg, 602.8kJ/kg, 26.33%, 0.004838m2]

4 A combined power and process plant is operating under steady flow conditions. The working fluid enters the system as superheated steam at a pressure of 30 bar, a

temperature of 400oC and a velocity of 90 m/s. The working fluid leaves the system as water having a specific enthalpy of 381kJ/kg and

a velocity of 10m/s. The exit point of the plant is 35m below the entry point. A heat loss of 100kJ/kg of fluid occurs during its passage through the plant. Determine for 1 kg of fluid flowing through the plant: (a) The enthalpy at entry (b) The change in kinetic energy (c) The change in potential energy (d) The work done by the fluid (e) The power output from the plant if the actual mass flow rate is 1.4 kg/s (f) The flow area of the entry pipe if the actual mass flow rate is 1.4 kg/s [Ans 3231kJ/kg, -4kJ/kg, -0.343kJ/kg, 2754.3kJ/kg, 3.856MW, 1545mm2]


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5 A steam turbine is supplied with steam at 1MN/m2 and 250oC through a duct of area 0.1m2, the mass flow rate being 5kg/s. The steam leaves the turbine at 50kN/m2 and 100oC, with a velocity of 130m/s. Heat losses amount to 10kJ/kg of steam.

Determine (a) entry velocity and (b) the power output of the turbine, neglecting any

changes in potential energy. [Ans 11.64m/s, 1.215MW] 6 The steam supply to an engine is at 7bar dry saturated, and the steam exhausts at 0.1bar

with a dryness fraction of 0.85. The entry and exit velocities are 30m/s and 170m/s respectively, and a heat loss of 12kJ/kg of steam occurs during its passage through the engine. The steam mass flow rate is 2kg/s.

Determine (a) the power developed by the engine assuming that potential energy

changes are negligible, and (b) the cross-sectional area of the exit pipe. [Ans 1.032MW, 0.1466m2] 7 Steam flows steadily at a rate of 2kg/s through a pipe of constant diameter. At entry to

the pipe, the pressure is 1.6MN/m2, the steam is dry saturated and has a velocity of 60m/s. At the pipe exit, the pressure is 1.5MN/m2 and the steam is 0.98 dry.

Determine (a) the pipe diameter, (b) the velocity of the steam at exit, and (c) the heat loss

rate from the pipe. [Ans 72.4mm, 62.6m/s, 80kW] 8 Steam enters a combined power and process plant at a pressure of 30bar and a

temperature of 400oC with a velocity of 90m/s and leaves as liquid at 1bar and 90oC with a velocity of 10m/s. The exit point is 35m below the entry point and the flow rate is 5040kg/hour. The heat loss rate from the plant is 140kW.

Determine for the plant (a) the power output (b) the exit pipe flow area. [Ans 3.862MW, 145mm2]


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9 Steam enters a nozzle with negligible velocity at a pressure of 15bar and a dryness fraction of 0.95. At the nozzle exit the steam has a pressure of 10bar and a dryness fraction of 0.94.

Calculate the velocity of the steam at the exit from the nozzle, assuming that the flow through the nozzle is adiabatic.

[Ans 274m/s] 10 Steam enters a pipe at 7bar and 250oC and flows steadily along the pipe at constant

pressure. If heat is lost to the surroundings at a steady rate from the steam as it flows along the pipe, at what temperature will water droplets begin to form in the steam?

Calculate the heat lost per kg of steam flowing, up to the point where the water droplets first begin to form, stating any assumptions that you must make.

[Ans 165oC, 191kJ/kg]

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thermodynamics 2 e7

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